Induction Balance Stuff - Single/Multi Frequency Response, GB, Disc, Measurements, Ideas, Fun, etc.

[Aziz]

LTspice file (zipped)..

[Aziz]

Hi all,

so lets go to the ultimate ground breaking dual-frequency KISS detector solution.
You will be satisfied with the detection depth and high mineral ground handling.

Instead of operating the detector with a mono coil, we are going to use an IB-coil.
Operated on two frequencies f1 and f2.
We have 4 I/Q vectors for processing:
- IQ_TXf1,
- IQ_TXf2,
- IQ_RXf1,
- IQ_RXf2
We just add two protection diodes for the RX coil.

This is the final schematics:

You will really get a good detection sensitivity and depth with it.
And there is no need for an amplifier. There is enough target signal there.

But the detector software is tricky. All the power lies in the coding.

Aziz

[Aziz]

Hi all,

my new breadboard is ready to fire it up.



On a single frequency resonance, it is the most sensitive hot ground/ferrite detector ever.
The circuit simulation did tell it.
The nulling works perfect and I can achive 0.000000 V on my virtual ground node.

Instead of nulling, a dual frequency mode is more interesting. BTW, there is no need for chirp modulation. Two frequencies are enough.
Left channel: Frequency f1
Right channel: Frequency f2
f2-f1 = 100 Hz (or so)

The two frequencies deliver enough information to cope with ground balance and target response processing.
The sensitivity is less than compared to single frequency response (the transmit spectral energy is distributed on two frequencies).

Not bad for the simplicity of the design (total 3 capacitors and a mono coil TX, I'm using two 15 nF parallel resonant capacitors in the picture above).
The dual-frequency detector can be realised on a mono coil. But it can't concur with other detectors, which avoid much larger x-response.

Aziz

[Aziz]

Hi Paul,

Aziz ... you are chasing the wrong method ...read the relevant minelab patents. tempco is not the caps its the coil itself.

The static null holds only at one operating point because Vn and Ph are open-loop constants. Ground loading perturbs the coil in two ways at once: soil susceptibility adds reactive ΔL (shifts tank resonance) and eddy-current loss adds ΔR (drops Q). Both move the TX-current phasor that the null was trimmed against, and nothing in the circuit chases it.

The damaging part is the Q multiplication. For a fractional inductance change δ = ΔL/L, the resonance moves by ½δ in frequency, and near resonance the phase slope is ~2Q, so the TX current phase rotates by roughly

Δφ ≈ Q · (ΔL/L)

Your .step p list 1.0 1.01 1.02 is precisely δ = 0%, 1%, 2%. At Q = 100 a 1% ground-induced ΔL is already ~1 rad ≈ 57° of phase walk on the feedthrough — the null is simply gone. Even a modest Q = 20 gives ~11°, far beyond what any fixed Vn/Ph can hold. So the very thing that makes the tank efficient (high Q) is what makes the static null fragile against ground.

Tempco hits both axes independently: Ct and the former's CTE drift the resonance (so more Δφ), and copper's +0.39%/°C drifts R, hence Q, hence the feedthrough amplitude — ~12% over a 30°C swing, directly an amplitude null error since Vn is fixed.

I know, I will get huge headache with it.
But I am planning to look at a narrow bandwidth chirp modulation and see what happens. Even I can't null it.
So away from single frequency to multi-frequency processing.
I am curious.
Aziz

[moodz]

Aziz ... you are chasing the wrong method ...read the relevant minelab patents. tempco is not the caps its the coil itself.

The static null holds only at one operating point because Vn and Ph are open-loop constants. Ground loading perturbs the coil in two ways at once: soil susceptibility adds reactive ΔL (shifts tank resonance) and eddy-current loss adds ΔR (drops Q). Both move the TX-current phasor that the null was trimmed against, and nothing in the circuit chases it.

The damaging part is the Q multiplication. For a fractional inductance change δ = ΔL/L, the resonance moves by ½δ in frequency, and near resonance the phase slope is ~2Q, so the TX current phase rotates by roughly

Δφ ≈ Q · (ΔL/L)

Your .step p list 1.0 1.01 1.02 is precisely δ = 0%, 1%, 2%. At Q = 100 a 1% ground-induced ΔL is already ~1 rad ≈ 57° of phase walk on the feedthrough — the null is simply gone. Even a modest Q = 20 gives ~11°, far beyond what any fixed Vn/Ph can hold. So the very thing that makes the tank efficient (high Q) is what makes the static null fragile against ground.

Tempco hits both axes independently: Ct and the former's CTE drift the resonance (so more Δφ), and copper's +0.39%/°C drifts R, hence Q, hence the feedthrough amplitude — ~12% over a 30°C swing, directly an amplitude null error since Vn is fixed.

[Aziz]

Hi all,

I have replaced R1 with a capacitor C1. It is more power efficient and should not produce temp drifts.
Same principle.

[Aziz]

...
and this will allow us to operate the detector at different modes:

Mono coil plugged:
-> mono coil VLF

IB-coil plugged:
-> Mode 1: mono coil VLF (TX only)
-> Mode 2: IB-coil VLF (RX only)
-> Mode 3: mono + IB-coil VLF (TX, RX, dual channel)

[Aziz]

Hi all,

this is becoming very very interesting now.

A true two channel VLF processing is possible in conjuction with high TX current (up to 500 mA peak) and IB-coil (with IB RX coil).
Channel 1: TX coil processing (at the nulling node, with some residual signal level).
Channel 2: RX coil processing.
As we can't take the TXref signal, we are referencing to the internal generated reference.
There should be enough ground balance information to cope with high ground mineralisation. Well, I hope.

[Aziz]

BTW,

the last mono coil VLF with active nulling has some advantages:
- Transmit energy balanced to two channel outputs (left, right).
- Transmit power doubles (both channels used) -> high coil current, high coil voltage. We can push up the transmit power if required by increasing capacitor value Cs (1 nF).
- Easy to find resonance frequency and easy to null:
1. Set nulling VL channel to 0 V (no signal), transmitter works with half power
2. Adjust VR channel frequency for resonance, max Lin-Signal -> resonance frequency found
3. Set the found resonance frequency to VL channel and adjust voltage level and phase shift (180 + x) until Lin-Signal is minimum (near 0 V). Some residual signal level is required. No need to perfectly match to 0.000000 V.
4. Ready to go.

I have to build a new bread board to test the circuit. I may add a voltage divider for the TX reference signal (Rin).

Aziz

[Aziz]

Hi all,

well ok. The problem with the high impedance signal path has been solved and there is no j-fet amplifier required anymore.
Also the mixer has been radically simplified. It's really a KISS masterpiece now.
The mixer has been reduced into one resistor (R1).

Provided that, the ground mineralisation isn't much, the following basic LC-tank with active nulling can be used and the signal source can be directly fed to the input of the sound card.
It is now a quite low impedance path (impedance aprox. R1/2, so much lower than 10 k Ohm).

Aziz

PS: And no external battery required anymore too.

[Aziz]

Well ok,

the nulling won't work on chirp modulation. Too complex issue.
The TX voltage varies still too much for 0%, 1% and 2% TX inductance increase.

It is really nice, that LTspice has a modulate module to make chirp modulation possible. The AI told me how to use it.

Aziz

[Aziz]

Hi all,

there might be a very tricky solution to the huge ground reponse on mono coil vlf with active nulling however.

If we would expect 1 .. 2 % TX inductance increase due to heavy ground mineralisation, a chirp modulation of the TX driving and nulling signal from Frequency f1 to f2,
where f2 is the resonant frequency with 0% TX inductance change and f1 the resonant frequency with maximum expected TX inductance change (1 .. 2%).

It could be worth to test this idea. This is the last chance for mono coil vlf with improved performance.
Aziz

[Aziz]

Hi all,

after making extensive circuit simulations, the active nulling on mono coil vlf won't work on heavy ground mineralization.
The ground mineralisation response would be of order 2, 3, or 4 and more (order = 10^x) compared to target response.
This would overload the pre-amp input. Active nulling does not make sense anymore.

It would be possible to realise the idea on mild ground however. To some level of ground mineralisation of course.

3 lecture lessons learned:
- How to design and wind very good signal transformers
- Checked my coil software: It is calculating correct responses.
- Need to do more circuit simulations: It saves a lot of time.

Aziz

[Sergey_P]

Hi, Alex
There are changes, but not at the detector frequencies

[alex.drozdov]

Hi, alex
Changing the signal frequency changes the impedance due to changes in the reactive component's resistance. The active component (R) is independent of frequency.
Energy is expended on the active component (R), so matching between the source and load occurs at the R level.

Hi, Sergey_P!
As i know Skin and proximity effects increase active resistance with increase signal frequency.
Classic MD coil is tight wounded with many turns so this coil should have big difference of active R on DC and AC on khzs , for freqs around 20khz skin effect not too much but proximity effect should give great increasing of active R. On 50khz skin and proximity should give great increase of R.
More active R - more voltage need to give enough current for enough magnet flux, more voltage is sign of increasing real impedance of TX coil and electricity part of EM field generated more in this coil, if compare with some current but on low acitve R coil..
Last coils from minelab equinox e.t.c. has very low turns coil and with very low active R (litz on rx and tx ) I think in minelab it did for good matching coils with low impedance soil (salted soils, highly minerilized soils in australia ?)

skin and proximity effects should be applicable for soil too.